I recently made a similar switch. I had a late 07 BlackBook 2.2. I had upgraded to 4gb ram and 320HD.
Primary use for the computer was an internet machine for the wife. I use my iPad around the house so she would use the MB. It ran great, had no issues and was overall a wonderful machine. My only gripe, it was heavy when compared to using my iPad.
I listed it on eBay and got well over what I thought was market value for it and replaced it with a 11.9 ultimate. Its an awesome machine. The size, the start up time everything. The only thing I miss is some HD space but I've moved all the phones, pods and pads to sync with my Mini (which has everything on it) and all is well.
Did I need a new computer, no. Did I want a new computer, yes. So I bought it. Sounds like you're in the same position.